Boc signal acquisition and tracking method and apparatus

ABSTRACT

A BOC signal acquisition and tracking apparatus and method. In the present invention, at least a BOC signal, a BOC-cos signal and a PRN coded signal are generated for a received signal. Depending on application condition (e.g. acquisition mode or tracking mode), autocorrelation of the BOC signal is combined with cross-correlation of the BOC signal and one of the BOC-cos signal and the PRN coded signal to generate a proper combined correlation result.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to processing of binary offset carrier(BOC) modulated signals (simply referred to as BOC signal hereinafter),more particularly, to a method and apparatus for processing BOC signalsin acquisition and tracking modes of a satellite navigation receiver.

BACKGROUND OF THE INVENTION

Nowadays, more than one Global Navigation Satellite System (GNSS) isavailable. A receiver supporting multi-specification LBS (location basedservice), wireless multimedia communication and broadcasting signals isbecoming an expectation. Take multi-specification LBS as an example,such a receiver able to support multi-mode receiving for GNSS signalscan enhance locating precision and access to more services. Among theGNSS systems, different signal frequency bands support differentservices. As more and more bands need to be supported, band overlappingoccurs.

GPS is the U.S. navigation satellite system, which is a network ofsatellites continuously transmits high-frequency radio signals. Thesignals carry time and distance information that is receivable by a GPSreceiver, so that a user can pinpoint the position thereof on the earth.Galileo, the emerging European satellite navigation system, offershigher signal power and more robust modulation that will enable users toreceive weak signals even in difficult environments. When combined,Galileo and GPS will offer twice the number of satellite sources ascurrently available. This provides redundancy as well as greateravailability for the user. The combination of GPS and Galileo basicallyhas four bands, excluding SAR (Safe and Rescue) service. GPS and Galileosystems share some signal bands. That is, GPS and Galileo share somecentral frequencies and send signals on the same ones of carriers. Forexample, GPS L1 and Galileo E2-L1-E1 share the same band. To reduceinter-system and intra-system interference, specific modulation schemesare required. Binary offset carrier modulation (hereinafter simplyreferred to as “BOC”) is a widely used method.

The BOC modulation is done by multiplying a pseudo-random noise (PRN)spreading coded signal (simply referred to as PRN coded signalhereinafter) with a square wave subcarrier (SC). The SC has a frequencywhich is multiple of the code rate of the PRN spreading code. FIG. 1 isa waveform diagram showing the BOC modulation. The BOC-sine (simplyreferred to as BOC) signal is generated by mixing a SC-sine and a PRNcoded signal, while the BOC-cos (also referred to as QBOC, where Qindicates “quadrature-phase”.) is generated by mixing an SC-cos and thePRN coded signal.

The BOC signal has a symmetric split spectrum with two main lobesshifted from the center frequency by the frequency of the subcarrier.The characteristics of the BOC signal are dependent on the spreadingcode chip rate, the subcarrier frequency, and the subcarrier phasingwithin one PRN code chip. The common notation for a BOC-modulatedsignals in the GNSS field is represented as BOC(f_(c), f_(s)), wheref_(c) is the code chip rate, and f_(s) is the frequency of thesubcarrier. Both f_(c) and f_(s) are usually represented as a multipleof the reference frequency 1.023 MHz. Therefore, the BOC signal can alsobe represented as BOC(n,m), where n is the multiple of 1.023 MHz; forthe PRN code chip rate f_(c), and m is the multiple of 1.023 MHz for thesubcarrier f_(s).

For satellite signal navigation, the BOC signal is preferably applied intracking under white noises. Such scheme provides better inherentmultipath mitigation compared to the spreading code alone. However, BOCscheme makes acquisition and tracking more difficult due to a multiplepeak autocorrelation phenomenon. The presence of the subcarrier in theBOC signal introduces secondary peaks in a range of −1/+1 chip in BOCautocorrelation. FIG. 1 is a diagram showing autocorrelation ofBOC(1,1). That is, BOC(1,1) correlates with BOC(1,1). As shown, thereare two troughs at both sides of the main peak in the middle. Tocalculate correlation power, square of correlation is usually used.Accordingly, the two troughs will cause two secondary peaks in view ofcorrelation power. Such secondary peaks may cause a problem of mis-lock.That is, a receiver may lock the secondary peak rather than the mainpeak, and therefore resulting in erroneous tracking. A significantdeviation of approximately 150 m would occur in the range measurement.Such an error is unacceptable in navigation.

In addition, the width of the main lobe (main peak) of the BOCcorrelation result influences the performance of the receiver inacquisition and tracking. If the main lobe is narrow, it is good fortracking and position because a more accurate code phase can be tracked.However, a narrow main lobe makes it difficult to acquire the signalbecause the narrow correlation function leads to a finer code phasesearching space, which needs longer acquisition time.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a BOC signalacquisition and tracking apparatus. The apparatus comprises a carrierunit generating a carrier; a code unit generating a plurality ofsubcarriers including at least a BOC subcarrier, a BOC-cos subcarrier,for example, as well as a PRN code, and outputting the BOC subcarrier aswell as one of BOC-cos subcarrier and the PRN code; and a code delayestimator receiving a signal, removing a carrier component from saidsignal by using said carrier from the carrier unit, generating a BOCsignal for the received signal by using the BOC subcarrier andgenerating a BOC-cos signal or a PRN coded signal for the signal byusing one of the BOC-cos subcarrier and the PRN code, calculating anautocorrelation of the BOC signal and a cross-correlation of the BOCsignal and one of the BOC-cos signal and the PRN coded signal, andcombining said autocorrelation and said cross-correlation to generate acombined correlation. The apparatus has a controller controlling thecarrier unit, the code unit and the code estimator. The controllercontrols the code unit to output the BOC-cos subcarrier or the PRN code.For example, in signal acquisition mode, the controller controls thecode unit to output the BOC subcarrier and BOC-cos subcarrier; while insignal tracking mode, the controller controls the code unit to outputthe BOC subcarrier and the PRN code. Accordingly, the code estimator canproperly generate a combined correlation based on built-in algorithms.

Another objective of the present invention is to provide a BOC signalacquisition and tracking method. The method comprising receiving asignal; generating a carrier; generating subcarriers including at leasta BOC subcarrier, a BOC-cos subcarrier, for example, as well as a PRNcode; outputting the BOC subcarrier and selecting to output one of theBOC subcarrier and said PRN code; removing a carrier component from thereceived signal by using said carrier; generating a BOC signal for thesignal by using the BOC subcarrier; generating one of a BOC-cos signaland a PRN coded signal for the signal by using one of the BOC-cossubcarrier and the PRN code; calculating an autocorrelation of the BOCsignal; calculating a cross-correlation of the BOC signal and one of theBOC-cos signal and the PRN code signal; and combining theautocorrelation and the cross-correlation to generate a combinedcorrelation. For example, in signal acquisition mode, the BOC subcarrierand BOC-cos subcarrier are output; while in signal tracking mode, theBOC subcarrier and the PRN code are output. Accordingly, the combinedcorrelation can be properly generated based on built-in algorithms.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described in details inconjunction with the accompanying drawings.

FIG. 1 is a waveform diagram showing generation of BOC and BOC-cossignals;

FIG. 2 shows correlation result of BOC(1,1) autocorrelation;

FIG. 3 shows correlation powers of cross-correlation of BOC(1,1)/PRNcode, autocorrelation of BOC(1,1) and a combined correlation of the bothwith a specific coefficient;

FIG. 4 shows correlation powers of autocorrelation of BOC(1,1)cross-correlation of BOC(1,1)/QBOC(1,1), and a combined correlation ofthe both with a specific coefficient; and

FIG. 5 is a block diagram showing a BOC signals acquisition and trackingapparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, secondary peaks of a BOC signal due to a subcarrieris undesirable. A method to remove the secondary peaks is a combinedcorrelation function. As shown in FIG. 3, correlation power ofautocorrelation of BOC(1,1) (i.e. BOC(1,1) correlates with BOC(1,1))provides curve with a main peak and two side peaks (or secondary peaks).In addition, cross-correlation of BOC(1,1) and PRN code provides a curvewith two side peaks but without the main peak. By properly combiningautocorrelation of BOC(1,1) (i.e. BOC(1,1) BOC(1,1)) andcross-correlation of BOC(1,1) and PRN code (i.e. BOC(1,1)/PRN), sidepeaks can be effectively removed. The correlation combination functioncan be expressed as

R _(combi) =R ² _(BOC/BOC)(τ)−α×R ² _(BOC/PRN)(τ)  (1)

where τ is code delay in chips, and α is a variable coefficient. In FIG.3, BOC (1,1) is used and α is 1.4. As can be seen, the combinedcorrelation power curve has a narrower main lobe with a significantlyhigh peak, but has no side peaks. Such a function is preferably appliedin signal tracking for the sake of better accuracy.

To rapidly acquire a signal, it is preferred that the correlation curvehas a wide main lobe. A method to obtain a wide correlation functionwhile reduce side peaks is combining BOC autocorrelation andcross-correlation of BOC and BOC-cos (QBOC). FIG. 4 shows correlationpower curves of BOC(1,1)/BOC(1,1) and BOC(1,1)/QBOC(1,1). By adding theboth with a proper coefficient, a resultant combined correlation with awide main lobe can be obtained. The combining function can be expressedas:

R _(combi) =R ² _(BOC/BOC)(τ)+β×R ² _(BOC/QBOC)(τ)  (2)

where τ is code delay in chips, and β is a variable coefficient. In FIG.3, BOC(1,1) and QBOC(1,1) are used and β is 0.8. As shown, the combinedcorrelation has a wide-shaped main lobe. Although there still are sidepeaks existing, the side peaks have been somewhat reduced and smoothed.Such a function is suitable for signal acquisition.

It is noted that in either correlation combination, the coefficient (α,β) is variable as desired.

FIG. 5 is a block diagram schematically showing a BOC signal acquisitionand tracking apparatus in accordance with the present invention. Theapparatus can be implemented as a portion of a GNSS signal receiver(e.g. a GPS receiver). The apparatus receives incoming IF data from anRF frond end of a GNSS receiver, for example. Reference number 10indicates a carrier unit, which provides a carrier signal to carriermixers 102 and 104 to remove IF component from the data. The carriersignal can be generated by a local oscillator, which is implemented by acarrier numeral controlled oscillator 12. Reference number 14 indicatesa phase shifter. The IF-removed signal in I and Q channels are then fedto mixers 202 and 204, 206 and 208, respectively. Block 20 is referredto as a code unit. In accordance with the present invention, the codeunit 20 comprises a code numeral controlled oscillator 22 for providinga code signal, a PRN code generator 24 receiving the code signal fromthe code NCO 22 to generate the PRN code, and a pulse shaping unit 25.The pulse shaping unit 25 receives the PRN code to generate a BOCsubcarrier and BOC-cos subcarrier by using the PRN code. The subcarriersare generated by a subcarrier generator 252 in the pulse shaping unit25. The pulse shaping unit 25 outputs the BOC subcarrier as well as oneof the BOC-cos (QBOC) subcarrier and the PRN code. The pulse shapingunit 25 has a multiplexer 254 receiving the BOC-cos (QBOC) subcarrierand the PRN code, and selecting to output one of the both.

The BOC subcarrier is provided to the mixers 202 and 206, so that BOCsignal is generated in I and Q channels. The selected output from themultiplexer 254 is fed to the mixers 204 and 208. When the selectedoutput is the BOC-cos subcarrier, a BOC-cos (QBOC) signal is generated.When the selected output is the PRN code, a PRN coded signal isgenerated. The outputs of the mixers 202, 204, 206, 208, which arereferred to as code mixers, are fed into integration and dump units 302,304, 306, 308, respectively, to be integrated and dumped. Then theintegrated results from the integration and dump units 302, 304, 306,308 are fed to a combination unit 40. The combination unit 40synthesizes the integration results by combining the integration resultsfrom units 302 and 304 to obtain combined correlation in I channel andcombining the integration results from units 306 and 308 to obtaincombined correlation in Q channel. The combination unit 40 combines theintegration results based on the equations (1) or (2). In addition, thecoefficient α or β is determined in the combination unit 40 in thepresent embodiment. However, the coefficient α or β can also beexternally provided to the combination unit 40.

An output of the combination unit 40 is fed to a discriminator 50, whichoutputs a tracking error from the received correlation to feed back tothe carrier unit 10 and code unit 20, so that these units can executeproper adjustments. The mixers 102, 104, 202, 204, 206, 208, integrationand dump units 302, 304, 306, 308, combination unit 40 and discriminator50 compose a code delay estimator 100.

The apparatus in accordance with the present invention further has acontroller 60. The controller 60 controls the carrier NCO 12, the codeNCO 22 and the multiplexer 254. For example, the controller 60 controlsthe multiplexer 254 to output the BOC-cos signal in signal acquisitionmode while controls the multiplexer 254 to output the PRN code in signaltracking mode. The controller 60 can receive an external command andcontrols the respective units accordingly. In another embodiment, thecoefficient α or β used in the combination unit 40 is determined by thecontroller 60.

Although the BOC-sine signal (BOC signal), the BOC-cos signal (QBOCsignal) as well as PRN code are described in the embodiment, othersignals combination, such as a BOC signal with a BOC harmonic signaland/or BOC-cos harmonic signal thereof, can be used. Here, the so calledBOC harmonic signal indicates BOC of a multiple of f_(s), For example, adouble frequency harmonic subcarrier of the BOC subcarrier isrepresented as BOC-sin(2f_(s)), and a double frequency harmonicsubcarrier of the BOC-cos subcarrier is represented as BOC-cos(2f_(s)).The rest can be deduced accordingly.

In addition, in the above embodiment, the code unit 20 outputs PRN code,BOC subcarrier and BOC-cos subcarrier. However, more than the PRN codeand the above two subcarriers can be generated and output by the codeunit 20, such as harmonic of the BOC and BOC-cos subcarriers. Themultiplexer 254 can outputs selected one or more among the PRN code anda plurality of subcarriers under the control of controller 60.

While the preferred embodiment of the present invention has beenillustrated and described in details, various modifications andalterations can be made by persons skilled in this art. The embodimentof the present invention is therefore described in an illustrative butnot in a restrictive sense. It is intended that the present inventionshould not be limited to the particular forms as illustrated, and thatall modifications and alterations which maintain the spirit and realm ofthe present invention are within the scope as defined in the appendedclaims.

1. A BOC signal acquisition and tracking apparatus comprising a carrierunit generating a carrier; a code unit generating a plurality of BOCsubcarriers and a code, and outputting a specific one of the BOCsubcarriers, and also outputting at least one of the remaining BOCsubcarrier and said code; and a code delay estimator receiving a signal,removing a carrier component from said signal by using said carrier fromthe carrier unit, generating a plurality of BOC signals for the receivedsignal by using the outputs from the code unit, calculating anautocorrelation of a specific one of the BOC signals and across-correlation of each remaining BOC signal with the specific BOCsignal, and combining said autocorrelation and each cross-correlation togenerate a combined correlation.
 2. The apparatus of claim 1, whereinsaid specific one of the BOC subcarriers comprises a BOC subcarrier. 3.The apparatus of claim 2, wherein the other BOC subcarrier comprises aBOC-cos subcarrier with respect to the BOC subcarrier.
 4. The apparatusof claim 2, wherein the remaining BOC subcarrier comprises a harmonic ofthe BOC subcarrier.
 5. The apparatus of claim 2, wherein the remainingBOC subcarrier comprises a harmonic of a BOC-cos subcarrier with respectto the BOC subcarrier.
 6. The apparatus of claim 1, wherein said code isPRN code.
 7. The apparatus of claim 1, further comprising a controllercontrolling said code unit to output one of the remaining BOC subcarrierand said code.
 8. The apparatus of claim 1, wherein said code unitcomprises a code generator for generating said code, a subcarriergenerator for generating the BOC subcarriers, and a multiplexerreceiving said remaining BOC subcarrier and said code, and outputtingone of said remaining BOC subcarrier and said code.
 9. The apparatus ofclaim 1, wherein said code estimator comprises a combination unit, saidcombination unit generates said combined correlation by squaring saidautocorrelation of said specific BOC signal, squaring saidcross-correlation of said specific BOC signal and said code, andsubtracting the cross-correlation square multiplied by a coefficientfrom the autocorrelation square when the code estimator outputs saidcode.
 10. The apparatus of claim 1, wherein said code estimatorcomprises a combination unit, said combination unit generates saidcombined correlation by squaring said autocorrelation of said specificBOC signal, squaring said cross-correlation of said specific BOC signaland said remaining BOC signal, and adding the cross-correlation squaremultiplied by a coefficient to the autocorrelation square when the codeestimator outputs said remaining BOC signal.
 11. A BOC signalacquisition and tracking method comprising steps of: receiving a signal;generating a carrier; generating a plurality of BOC subcarriers and acode; outputting a specific one of said BOC subcarriers and also saidcode; removing a carrier component from said signal by using saidcarrier; generating a plurality of BOC signals for said signal by usingsaid outputs of the outputting step; calculating an autocorrelation of aspecific one of the BOC signals; calculating a cross-correlation of eachremaining BOC signal with the specific BOC signal; and combining saidautocorrelation and each cross-correlation to generate a combinedcorrelation.
 12. The method of claim 11, wherein said specific BOCsubcarrier comprises a BOC subcarrier.
 13. The method of claim 12,wherein said remaining BOC subcarrier comprises a BOC-cos subcarrierwith respect to the BOC subcarrier.
 14. The method of claim 12, whereinsaid remaining BOC subcarrier comprises a harmonic of the BOCsubcarrier.
 15. The apparatus of claim 12, wherein said remaining BOCsubcarrier comprises a harmonic of a BOC-cos subcarrier with respect tothe BOC subcarrier.
 16. The method of claim 11, wherein said combinedcorrelation is generated by squaring said autocorrelation of saidspecific BOC signal, squaring said cross-correlation of said specificBOC signal and said code, and subtracting the cross-correlation squaremultiplied by a coefficient from the autocorrelation square when thecode estimator outputs said code.
 17. The method of claim 11, whereinsaid combined correlation is generated by squaring said autocorrelationof said specific BOC signal, squaring said cross-correlation of saidspecific BOC signal and said remaining BOC signal, and adding thecross-correlation square multiplied by a coefficient to theautocorrelation square when the code estimator outputs said remainingBOC signal.